$('.one-slide').each(function() { // the containers for all your galleries
    $(this).magnificPopup({
        delegate: 'a', // the selector for gallery item
        type: 'image',
        gallery: {
            enabled:true
        }
    });
});

$(document).ready(function() {
    $('#single-post .content .gallery-item').each(function() { // the containers for all your galleries
        $(this).magnificPopup({
            delegate: 'img', // the selector for gallery item
            type: 'image',
            gallery: {
                enabled:true
            },

            callbacks: {
                elementParse: function(item) { item.src = item.el.attr('src'); }
            }
        });
    });
});